Dowel vs. Mortise and tenon joints revisited:An ill-fated attempt at working with a jig manufacturer

Background

When I wrote my joint
strength test article, I concluded that the mortise and tenon joint
was slightly stronger than a multiple dowel joint.

I then went on to say that the multiple dowel joints I used were similar to those
produced by a particular doweling jig. This solicited an email from
the makers of the jig pointing out that I hadn't actually
used their jig, and that the joint geometry was not the same as theirs.
They asked that I remove all references to the name of their product, and that
in the event that my page should show up in a prominent position in searches for
their product, their lawyer would take the issue up directly with Google,
Yahoo, and other search engines.

Having little stake in the matter, I removed all
references to their jig and company name. If I was wiser, that would have
been the end of the story.

However, a few weeks later, I noticed on a public forum that discussed my tests the following
statement from one of the people working for the makers of the dowel jig (which shall
remain unnamed in this article):

"We will provide a [name removed] to the author for testing if the strength test can
be conducted using an optimally constructed [name removed] joint compared to an equivalent
m&t joint. We have done this already with Steve M. from the Canadian Woodworking forum."

I thought this would be an interesting test, and so contacted the makers of this dowel
jig again, and over the course of several emails, we agreed to cooperate on another
test, with the joint geometries nailed down ahead of time.

The specimens, as sent to me

We agreed to make both joints 1" deep. I would have preferred something like 1 1/4",
but the makers of the doweling jig wanted 1" or 2". I don't have a long enough router
bit for 2", so we settled on 1". In my previous tests, I had made the tenons almost
the full width of the piece, but this time we agreed to make the tenon no wider than the space occupied
by the dowels. Seeing that the dowels can't be placed all the way to the edge, this
made for a tenon that wasn't the full width of the stock.

I would have much preferred the mortised piece to be fairly thick, but the makers of the
dowel jig insisted that it be 3/4" (18mm). I figured this would not leave that much wood
on either side of the mortise, so I suggested using a 1/4" wide mortise against the 3/8"
wide dowels, which the makers of the dowel jig agreed to.
I'm pretty sure that the amount of remaining material, and likely my choice of 1/4" tenons
over 3/8" tenons turned out to probably be a key factor in how the tests turned out.
The makers of the dowel jig always test against a 3/8" tenon in 3/4" stock.

Joint geometry

We decided to do tests in 3 7/8 x 3/4" (72 x 18 mm) red oak. We settled for
a four dowel joint, with one inch dowel penetration. The dowel joints
were prepared by the makers of the dowel jig and sent to me. They also sent some
blank boards of the same stock for me to make mortise and tenon joints in. Wood can
vary considerably, but with all the wood from the same source, hopefully this
source of variability would be less.

I used the blank boards to make mortise and tenon joints with a comparable geometry.
1" penetration, and a tenon width of 2 3/8". I chose a tenon thickness of 1/4". With the stock
only 3/4" thick, it didn't make much sense to cut away that much of the material
to make the mortise.

The dowels used were pre-compressed hardwood dowels, 2" in length, with the
dowel penetrating into either piece by 1".

Given the width of the stock, it might have made more sense to make
a deeper joint, but the only other option for the pre-compressed dowels was to
use 4" dowels. That would have meant 2" in either side, but I didn't have
a long enough 1/4" router bit to cut comparable mortises. They wanted to
keep the joints symmetrical, so using unequal penetration on either side
wasn't an option.

The glue we used for the tests were WeldBond, and ProBond.

Test setup

This board and pivots evenly split the force between
a fixed point and my bathroom scale

Because my bathroom scale only reads up to 300 pounds, I split the force from
the jack between two points, so that only half of the force would press on the bathroom
scale. Each pound read by the scale would thus be two pounds applied to the
joint. With the pivot 1 foot from the joint surface, each bathroom scale pound
works out to two foot-pounds.

This pivot at the top of the jack ensured the force was
applied at the right distance

My jig applied the force one foot from the joint, whereas
the jig used by the makers of the dowel jig applies a the force
very close to the joint (roughly 2"), resulting in stress on the
joint that has a substantial shear-force component.

Real furniture joints tend to get stressed by more moderate forces applied at
greater distance, so I like my tests more. Think of
the stresses on the joint between the piece that makes up the chair back and legs
and the part holding the seat when a fat guy leans back on a chair. The stress
is from force applied a fair distance from the joint itself.

Running the tests

For each joint tested, the wood cracked open before the joint
itself began to separate

For each joint, I pumped the jack until the pieces of the joint itself physically
separated to the point where it was clear that the force required was not
going to increase anymore.

Because the vertical piece was relatively thin with respect to the joint,
each joint initially split open on the vertical piece before the
joint itself started to yield.
This problem could have been avoided by using a much thicker vertical
piece, as I had done in my previous tests,
but the makers of the dowel jig wanted both pieces to be 3/4" thick.

A dowel joint, after testing. Much like the joint in the
pervious image, this joint initially cracked open in the vertical member.
Once the force was removed, this crack completely closed up again

A failed mortise and tenon joint. A remnant of the gap is still visible,
though it had mostly closed once the force was removed.

For all but one of the pieces, the force required to
actually separate the pieces was greater than the original force to crack
the post apart.

As it was, overall, the dowel joints had a higher force for the initial
cracking open, but a slightly lower force to subsequently separate the joint.

It's debatable which force is the actual point of failure of the joint.
The post splitting open certainly is a point of failure. But if the force
was removed after that point, the gap that opened up closed again, and nobody would
be the wiser that this joint had actually "failed". The joint shown at left
had actually cracked open much like the joint in the previous image. But once
the dowels let go, that crack closed up again, so that crack is not even visible
in the photo.

But the secondary breakage is also iffy. By the point that the dowels or tenon
actually let go from the wood they were in, everything was quite deflected.
There was also variability where the wood actually split open. The closer
it split to the joint, the less wood there was left to hold. So using
the final separation force may not be that realistic either.

It always amazes me that when woodworking magazines do joint tests, that they
don't go into this sort of thing. A nice final number certainly sounds better
than to muddy the water with a lot of analysis and discussion. I suspect
tests done in woodworking magazines often have similar issues, but I imagine
they figure people just want an answer, and not a lot of discussion about
the validity of the results.

I figured I should present both the force of the initial cracking open, and the
force required for final separation in my results, but this is where
I started running into problems with the manufacturer.

The makers of the dowel jig wanted only the forces
of the initial cracking open to be considered as the results and all other
numbers to be omitted from the report, which would make the dowel joint the winner.
I didn't feel that this would be realistic. It's normal to
test a joint at least to the point of maximum force. The initial cracking
open wasn't even breaking the joint itself, so was it even a measure of joint
strength? Certainly, the cracking open force, on average, was larger for
the dowel joints. This could be because the dowel joints cut away a bit less
of the wood of the vertical piece, thus weakening it less.

Of course, we each had out own motivations and biases. The makers of the dowel jig take pride
in stating that their joints are stronger than mortise and tenon joints
and were probably looking for another test to confirm this assertion.
I don't sell any dowel jigs, but I do make a little bit of money
selling
slot mortising machine plans and a tenon jig plans.
so I wasn't particularly interested in making mortise and tenon joints look bad.

And so perhaps it's no surprise that when the makers of the dowel jig test
their joints against mortise and tenon joints, the dowel joint wins, whereas when
I test it, the mortise and tenon joint does a little better.

Of course, if I wanted to, manipulating the results would be trivially easy.
With the variability of wood, all one has to do is select the right pieces and
the right results to make the results conclusively go one way or the other.

The numbers

Joint

Glue

Wood splitsforce

Pullout afterwood splits

Maximum force

Dowel

ProBond

135

80 (excluded)

135

Dowel

ProBond

115

110

115

Dowel

WeldBond

105

105

105

Dowel

WeldBond

130

135

135

Average dowel:

121

116

123

Mortise & Tenon

WeldBond

95

140

140

Mortise & Tenon

ProBond

110

135

135

Average 1/4" mortise & tenon:

103

138

138

Forces are in pounds read off of my bathroom scale. Each pound represents
two foot-pounds of moment applied to the joint. Two foot-pounds corresponds to 2.7
Newton-meters of bending moment, so 100 pounds in this table would be 270 Newton-meters.

It's hard to come to any definite conclusion with these numbers. Given the variation
in joints of the same type, the difference in failure force is not enough to
draw any statistically significant conclusion. And then the question is,
which is the number that counts? One could declare either joint type the winner.

I made two bandsaw cuts to the left and right of a mortise and tenon joint, at which point,
what was left just fell apart along the line that the wood that previously cracked open.

And here's a dowel joint split open the same way. It split just shy of
the ends of the dowels. The tapered ends of the dowels didn't provide
useful gluing surface, so the split is a little bit back from the tips of the dowels.
The mortise and tenon joint has the advantage there, with the glue surface extending
all the way into a flat-bottomed hole.

The aftermath

Given how the joints initially cracked open next to the joint, I figure how much wood
was left on either side of the joint is a key factor in this test.
The results were pretty close, and the way I see it, if I had made my tenons 3/8"
instead of 1/4" thick, I would expect that the remaining material around the
mortise would have been weaker and the dowel joint would have won. Certainly,
my tenons remained intact, so making the tenons thinner didn't weaken the joints.
My gut feeling is that a tenon needs to be at least five times longer than it is thick
before there is any risk of the tenon itself tearing apart.

I offered to run another set of tests with 3/8" wide tenons, but by this point,
the makers of the jig didn't want to have much to do with these tests anymore.
They also pointed out that they had used inferior
quality dowels for two of the tests and insisted that the two dowel joints
that failed at lower forces be omitted from the results.
I was not amused. Why would they deliberately use inferior quality dowels,
and why would they only tell me they did so after the tests were run?

This all struck me as a little strange. The dowel joints had actually performed quite
well in my tests, nearly as well as the mortise and tenon joints. Repeatability in
wood joint testing is generally poor, and with all the variability, it's hard to
say for sure that one joint is better than the other.
But the makers of the dowel jig insisted that their
joints are always stronger. We couldn't come to an agreement about the interpretation
of the results and I was unwilling to omit the samples and separation forces that
suggested mortise and tenon joints were stronger. And so, to be safe, I figure it's
best if I keep the makers of the dowel jig anonymous.

Given all this, I was glad that I only used some joints they sent me,
as opposed to accepting one of their fancy dowel jigs from them.
My thinking is that working with any manufacturer for review purposes
puts one in a difficult position, especially if one happens to
review a jig one doesn't like. So while corporate
sponsorship would be nice thing to have at times, life is much simpler without it.
At the rate I'm going, I doubt manufacturers are inclined to approach me :)

Further thoughts

For the type of geometry used in this test, I don't think it matters much if you
use a mortise and tenon joint or a multiple dowel joint. It's ultimately the wood
that breaks first.

What type of joint performs best will vary with a lot of factors. If this test
had been done with a very soft wood, the dowel joint would probably have
an advantage because four hardwood dowels would probably be stronger than
a softwood tenon.

Either joint could be improved on, but in different ways. The dowel joint
could have been made much deeper for a stronger joint.
The tenon joint could have been made a
little bit deeper without having to worry about what size pre-made dowels
are available. It most certainly could have been made the full width of the
material. Mortise and tenon joints also offer a bit more flexibility, because
one doesn't have to worry about sizing the joint for a specific number of dowels.

Ultimately, when you build furniture, it comes down to what you are set up
to do. If you have a doweling jig, a dowel joint can be faster. If you
have good mortise and tenon equipment,
the mortise and tenon is probably quicker. Both joint types are
strong enough for most applications, so it's best to base your choice
on how fast and accurately you can make them.
I prefer mortise and tenon joints more - with fewer surfaces, they were much
faster to glue up. Oh, did I mention I might be biased?